Preparation of indium alkoxides soluble in organic solvents

ABSTRACT

The invention relates to a process for preparing indium alkoxides soluble in organic solvents. 
     The preparation method consists in reacting an indium halide with a C 3  -C 20  alcohol in the presence of a base having a pka &gt;10 and a low nucleophilicity, in anhydrous medium, under inert gas and in the presence of polar organic solvents. 
     The invention allows to obtain with a high yield pure indium alkoxides.

The invention relates to the preparation of indium alkoxides. Moreparticularly, the invention allows to obtain pure indium alkoxides witha high yield, which are soluble in organic solvents and useful forcoating applications, for example to form conductive transparent thinfilms.

Few examples of the synthesis of indium alkoxides are given in theliterature. An article of J. Indian Chem. Soc, Vol LIII, Sep. 1976, pp.867-869 entitled "A study of Indium Alkoxides In(OR)₃ " describes apreparation of indium tri-isopropoxide by refluxing anhydrous indiumtrichloride with isopropanol in the presence of sodium isopropoxide. Byalcoholysis of indium tri-isopropoxide, other alkoxides can be obtained.However, indium isopropoxide and the resulting alkoxides are unlikely tobe pure and contain sodium, which is not acceptable for certainapplications where a high conductivity is desired; moreover, theseproducts are not very soluble in organic solvents and are very sensitiveto air, which makes them unsuitable for coating applications.

Other patents describe the synthesis of metal alkoxides consisting inreacting a metal halide with an alcohol in a basic medium.

For example U.S. Pat. No. 3,946,056 discloses a two step method toprepare stannic alkoxides. First stannic chloride is reacted with analkylamine, preferably a dimethylamine or a trimethylamine, then theproduct obtained is reacted with a tertiary alcohol. The said method isnot applicable to indium since the compound formed in the first step, aindium chloride/alkylamine complex is highly stable and does not furtherreact with the alcohol.

U.S. Pat. No. 4,681,959 discloses the preparation of metal alkoxidesinsoluble in organic solvents, such as methoxides. In an embodiment, thepreparation is a one-step process, wherein the metal halide is reactedwith the alcohol in the presence of a hydrogen halide acceptor which canbe an alkylamine, e.g. trimethylamine. The said hydrogen halide acceptorforms with the amine a compound soluble in reaction solvents while thealkoxide remains soluble in said solvents. Although all the examplesrelate to titanium alkoxides, the invention can be applied to thepreparation of indium alkoxides insoluble in organic solvents. Suchalkoxides are unsuitable for the preparation of coating compositionscomprising organic solvents.

In the prior art, coating compositions for forming indium oxide layerswere in general prepared from solutions comprising organic indiumcompounds; for example, the solution disclosed in U.S. Pat. No.4,391,742 which comprises 100 parts of an indium compound and 5 to 20parts of a stannic compound. The indium compound is an indium chelateobtained e g. by reacting indium chloride dissolved in an inert solventwith methyl acetoacetonate and n-butyl alcohol, in the presence oftriethylamine.

Therefore one of the objects of the present invention is the synthesisof indium alkoxides soluble in organic solvents, which can be used incoating compositions containing such solvents. It is also desired thatsaid alkoxides be pure, in particular that they do not containimpurities susceptible to decrease the conductivity of the final oxidelayers (for example Group IA metals). Moreover, said alkoxides should beobtained with a high yield for a low cost and to be easily isolated bysimple methods, e.g. crystallisation. Said alkoxides which areinherently highly moisture sensitive, should also be stabilizable tofacilitate their handling.

The preparation process according to the present invention allows toobtain indium alkoxides having all the above listed properties.

The synthesis according to the present invention consists in reacting ametal halide with an alcohol in basic medium, the said synthesisallowing the preparation of indium alkoxides with a wide range ofalcohols and bases.

The preparation method consists in reacting in a one-step process anindium trihalide with a mixture comprising a C₃ -C₂₀ alcohol and astrong base having a low nucleophilicity, in the presence of organicsolvents, in anhydrous medium and under an inert gas.

In addition to the desired indium alkoxides, by-products due to thepresence of the base are obtained. For example, if the halide is achloride, the base forms with HCl evolved during the reaction ahydrochloride. The said hydrochloride can be separated from the alkoxideby known methods involving for example differences of solubility inorganic solvents. However, in certain applications, such as coating toform indium oxide layers, the residual hydrochloride does not interferesince it is easily removed during the thermal treatments. The presenceof the base also generates as by-product, an indium chloride/basecomplex, which is partly soluble in the reaction medium and difficult toremove. However its formation can be avoided, when desired, by selectingappropriate bases according to certain criteria explained below. It isimportant to note that for certain applications, when it is present inan amount less than 30%, it has no deleterious effects; for example, ifconductive indium oxide films are to be formed, the said complex can actas a dopant.

In general, as indium halides, indium chloride or indium bromide areused.

Alcohols are selected so as to be soluble in the solvent used in thereaction, they must give alkoxides soluble in the coating solvents, andbe able to react with the indium halide. Preferably, alcohols having alow molecular weight, soluble in common reaction solvents are used.However, alcohols such as methanol or ethanol are not appropriate sincethey give insoluble alkoxides. The alcohols according to the inventionare C₃ -C₂₀ alcohols, preferably saturated cyclic or aliphatic alcoholsand most preferably cyclohexanol and primary alkanols.

For example, 2-propanol, 1-butanol, 2-butanol, 1-pentanol, cyclohexanoland 3-(trimethylsilyl)-1-propanol can be used, the preferred alcoholsbeing 1-butanol, 1-pentanol, cyclohexanol,3-(trimethylsilyl)-1-propanol.

According to the present invention, the bases must meet tworequirements: they must be strong, i.e. have a pka higher than 10 and atthe same time have a low nucleophilicity.

It is important to note that the nucleophilicity is a relative valuewhich depends on the electrophile, the solvent and steric effects. Anapproximate ranking of nucleophilicity can be found in the literature,for example in J. March Advanced Organic Chemistry, 3rd Edition, JohnWiley 1985, pp. 304-310.

With a base having a low nucleophilicity, the percentage of InCl₃ /basecomplex is very low, or even nil with bases known as having a very lownucleophilicity, as 2,6-di-t-butylpyridine (DTBP), or1,8-bis(dimethylamino)naphthalene (proton sponge). Conversely, the baseshaving a strong nucleophilicity such as ammonia, pyridine, imidazole,give percentages of complex close to 100%.

The bases both strong and having a low nucleophilicity can be selectede.g. among the bases cited in the Fluka Chemika-Biochemika Catalog, No16 of 1988-1989 under the heading "Strong and Hindered Nitrogen Bases".Examples of said bases are trimethylamine; triethylamine;triisobutylamine; 1,1,3,3-tetramethylguanidine (TMG);1,8-diazobicyclo[5.4.0]undec-7-ene (DBU);1,5-diazabicyclo[4.3.0]non-5-ene (DBN);2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine;1,4-diazabicyclo[2.2.2]octane (Dabco);N,N-diisobutyl-2,4-dimethyl-3-pentylamine;N,N-diisopropyl-3-pentylamine; 3-dimethylamino-2,4-dimethylpentane;N-ethyldicyclohexylamine; N-ethyldiisopropylamine;7-methyl-1,5,7-triazabicyclo-[4.4.0]dec-5-ene (MTBD);1,2,2,6,6-pentamethylpiperidine; tributylamine and proton sponge alreadycited.

Reaction solvents are organic solvents compatible with the alcohol andthe base selected. As solvent, the same alcohol as the alcohol formingthe alkoxide can be used or any polar solvent having a polarityindex >3.5, such as defined by L. Snyder in J. Chromatography 1978, 92,223-224, having no OH group and forming no stable complex with indiumchloride. Examples of solvents are chloroform, dimethylformamide,acetonitrile, tetrahydrofuran (THF), 1-butanol, dimethylformamide,pyridine, dimethylsulfoxide, acetonitrile, acetone, 1,4-dioxane,ethylacetate or mixtures thereof.

Preferred solvents are tetrahydrofuran, 1-butanol, dimethylformamide,acetonitrile or a mixture of THF/1-butanol.

It is to note that all the reactants should be strictly anhydrous.

Best results are obtained by adding indium chloride in organic solutionto a base/alcohol mixture, which minimizes the complex formation. Thereaction is carried out at temperatures between -50° C. and 80° C.,preferably about 0° C.

The following examples illustrate the invention.

EXAMPLE 1 Synthesis of Indium (III) 1-Tributoxide Using as BaseTriethylamine

Under argon, indium (III) chloride (35 mmol, 7.74 g) was dissolved in amixture of hexane (50 ml) and 1-butanol (25 ml), dried azeotropically,and concentrated to a final volume of 50 ml. This solution was addeddropwise to a solution of triethylamine (105 mmol, 10.62 g) in 1-butanol(25 ml) over 30 min at 0° C. Stirring was maintained for 30 min, afterwhich time the supernatant solution containing the alkoxide and theInCl₃ /base complex was removed from the precipated salts bydecantation. The salts were washed with 1-butanol (70 ml) to separatethe solid hydrochloride from the solution, and the combined solutionsevaporated under reduced pressure to leave the product as a viscousorange oil (9.5 g). NMR analysis of this product showed it to contain90% indium (III) 1-butoxide and 10% indium chloride/triethylaminecomplex.

EXAMPLE 2 Synthesis of Indium (III) 1-Butoxide Using as Base ProtonSponge

Proton sponge (14.55 mmol, 3.12 g) and anhydrous 1-butanol (29.1 mmol,2.16 g, 2.67 ml) were dissolved in anhydrous THF (20 ml). A solution ofanhydrous indium (III) chloride (dried using molecular sieves) inanhydrous THF (9.7 mmol, 10 ml of 0.97 M solution) was added dropwiseover 20 min at 0° C. Stirring was maintained for 18 h, whilst thetemperature was allowed to rise to 22° C. At the end of this time, thesolution was concentrated under reduced pressure and the resultantresidue containing the alkoxide and proton sponge hydrochloride waswashed with hexane (2×20 ml), dissolved in anhydrous dichloromethane (20ml), filtered to remove precipitated salts and evaporated under reducedpressure to leave the pure alkoxide/proton sponge hydrochloride mixtureas a pale cream coloured powder, (6.31 g). This material was shown notto contain any indium chloride/base complex. 1H NMR analysis indicatedthat this corresponded to a yield of 62% of desired alkoxide.

EXAMPLE 3 Synthesis of Indium (III) 1-Butoxide From Indium Bromide

Indium (III) bromide (38.78 mmol, 13.75 g) was dissolved in a mixture ofhexane (50 ml) and 1-butanol (50 ml), dried azeotropically, andconcentrated to a final volume of 60 ml. This solution was addeddropwise to a solution of triethylamine (116 mmol, 11.72 g) in 1-butanol(25 ml) over 30 min at 0° C. Stirring was maintained for 30 min, afterwhich time the hydrobromide was washed with 1-butanol (50 ml). Thesolvents were removed under reduced pressure. The product was obtainedas a viscous orange oil (12.3 g). NMR analysis of this product showed itto contain 89% indium (III) 1-butoxide and 11% indiumbromide/triethylamine complex.

COMPARATIVE EXAMPLE 4

The example illustrates the effect of the alcohol nature on the alkoxideyield.

Under strictly identical reaction conditions (argon gas, solvent:THF,base:triethylamine) the following results were obtained

    ______________________________________                                        Alcohol            Conversion to alkoxide %                                   ______________________________________                                        (1)  2-propanol        36                                                     (2)  1-butanol         92                                                     (3)  2-butanol         48                                                     (4)  1-pentanol        90                                                     (5)  cyclohexanol      82                                                     (6)  3-(trimethylsilyl)-1-propanol                                                                   74                                                     ______________________________________                                    

The example shows that the preferred alcohols according to the invention(2), (4), (5), (6) give better yields.

COMPARATIVE EXAMPLE 5

Similarly, indium (III)-cyclohexanoxides were obtained with variousbases in similar conditions (argon gas, solvent:THF,alcohol:cyclohexanol), unless it is otherwise specified.

The following table reports the alkoxide conversion percentages withcommon bases, as determined 1H NMR.

    ______________________________________                                        Bases         % alkoxide % InCl.sub.3 /base complex                           ______________________________________                                        (1)  ammonia       0         100                                              (2)  pyridine      0         100                                              (3)  imidazole     0         100                                              (4)  TMG          71         29                                               (5)  DBU          71         29                                               (6)  DBN          78         22                                               (7)  trimethylamine                                                                             75         25                                               (8)  triethylamine                                                                              82         18                                               (9)  proton sponge*                                                                             64          0                                               (10) diethylamine**                                                                             60         40                                               (11) DTBP***       0          0                                               ______________________________________                                         *The theoretical quatity of hydrochloride was also obtained                   ** 1butanol was used instead of cyclohexanol                                  ***DTBP (2,6di-t-butylpyridine) (pka 4.5 and having a low                     nucleophilicity). 1butanol as used instead of cyclohexanol. No conversion     was obtained.                                                            

It can be shown from the above results that only the bases according tothe invention, (4), (5), (6), (7), (8), (9) provide indium alkoxideswith high yields.

When it is desired to use said alkoxides for coating applications, e.g.to obtain indium oxide layers, the alkoxides can be stored beforecoating in alcohol under argon, e.g. if the alkoxide is indium (III) 1-butoxide in 1-butanol and stabilized with a small amount of acetic acidor acetylacetone.

We claim:
 1. A process for the preparation of indium alkoxides solublein organic solvents, wherein an indium halide is reacted in anhydrousmedium and under inert gas in the presence of polar organic solventswith a mixture comprising a) a C₃ -C₂₀ alcohol, b) a base selected amongstrong bases having a pka >10 and a low nucleophilicity.
 2. A processaccording to claim 1, wherein said indium alkoxide is indium chloride orindium bromide.
 3. A process according to claim 1, wherein said organicsolvent is tetrahydrofuran, 1-butanol, dimethylformamide, acetonitrile,or a mixture of tetrahydrofuran and 1-butanol.
 4. A process according toclaim 1, wherein the alcohol is a C₃ -C₂₀ saturated cyclic or aliphaticalcohol.
 5. A process according to claim 4, wherein the C₃ -C₂₀saturated aliphatic alcohol is a primary alkanol.
 6. A process accordingto claim 5, wherein said alcohol is 1-butanol, 1-pentanol, cyclohexanolor 3-(trimethylsilyl)-1-propanol.
 7. A process according to claim 6,wherein said organic solvent is tetrahydrofuran, 1-butanol,dimethylformamide, acetonitrile, or a mixture of tetrahydrofuran and1-butanol.
 8. A process according to claim 1 carried out in one step. 9.A process according to claim 8, wherein said organic solvent istetrahydrofuran, 1-butanol, dimethylformamide, acetonitrile, or amixture of tetrahydrofuran and 1- butanol.
 10. A process according toclaim 9, wherein the alcohol is a C₃ -C₂₀ saturated cyclic or aliphaticalcohol.
 11. A process according to claim 10, wherein the C₃ -C₂₀saturated aliphatic alcohol is a primary alkanol.
 12. A processaccording to claim 11, wherein said alcohol is 1-butanol, 1-pentanol,cyclohexanol or 3-(trimethylsilyl)-1-propanol.
 13. A process accordingto claim 12, wherein said organic solvent is tetrahydrofuran, 1-butanol,dimethylformamide, acetonitrile, or a mixture of tetrahydrofuran and1-butanol.
 14. A process according to claim 8, wherein said indiumalkoxide is indium chloride or indium bromide.
 15. A process accordingto claim 1, wherein the alcohol is a C₃ -C₂₀ saturated cyclic oraliphatic alcohol.
 16. A process according to claim 15, wherein the C₃-C₂₀ saturated aliphatic alcohol is a primary alkanol.
 17. A processaccording to claim 16, wherein said alcohol is 1-butanol, 1-pentanol,cyclohexanol or 3-(trimethylsilyl)-1- propanol.
 18. A process accordingto claim 17, wherein said organic solvent is tetrahydrofuran, 1-butanol,dimethylformamide, acetonitrile, or a mixture of tetrahydrofuran and1-butanol.
 19. A process according to any of claims 1-18, wherein saidbase is trimethylamine, triethylamine, 1,1,3,3,-tetramethylguanidine(TMG), 1,8-diazabicyclo undec-7-ene (DBU), 1,5-diazobicyclo non-5-ene(DBN) or 1,8-bis(dimethylamino)naphthalene (proton sponge).